In one known configuration of gas turbines engines, a number of discs, some comprising radially extending blades which are inserted to the discs, are provided to form a rotor. There are sets of discs for compressor blades and sets of discs for turbine blades. The respective sets of discs are retained by a turbine nut and a compressor nut respectively applied to one or two tension studs, the nuts and the studs are typically also used to apply a preload to tension the arrangement to ensure that all rotating parts are secure during operation of the turbine.
In current turbines, the rotor may be held together by a pair of tension studs. In the following one possible way how to assemble a compressor and a turbine is explained in a simplified manner. A first threaded end of the first stud may engage into a threaded bore in a shaft element of the rotor. A compressor disc then may be pushed axially into position and locked to the shaft element. Further compressor discs may additionally be pushed into position. Finally a threaded compressor nut may be engaged to a second threaded end of the first stud and tightened such that all compressor discs are secured to each other and the shaft element. For the turbine discs, a first threaded end of the second stud may engage in a threaded bore of the other end of the shaft element. Then turbine discs may be pushed axially into position from the opposite side and a threaded turbine nut may be applied to a second threaded end of the second stud and tightened such that all turbine discs may be locked to the shaft element.
During operation of the gas turbine engine stress may be experience in the studs. Furthermore stress may be not evenly distributed over all threads of the studs and the nuts with the consequence of local peak loads that could result in fatigue of the affected threads and consequently to potential failures.
In FIG. 1A, a threaded compressor stud (CS) is rotated into threaded engagement into a threaded bore in an intermediate shaft (INTS) and compressor discs (CD) are slid over the compressor stud (CS) from left to right during assembly. An inlet shaft (IS) is then mounted onto the compressor stud (CS) and a compressor pre-load nut (CN) threaded onto the compressor stud end. For assembly a hydraulic tool may be applied to stretch the stud (CS) and the compressor nut (CN) is tightened to engage the inlet shaft (IS) before the tool is removed. This retains the pre-load—which also can be called pre-tension—applied to the compressor stud (CS) via the nut (CN). The stretch required may be affected by relative thermal and mechanical expansion and contraction at different operating conditions of the stud (CS) and the clamped components, e.g. the compressor discs (CD).
FIG. 1B shows a turbine stud (TS) threaded into another axial end of the intermediate shaft (INTS). Then—not yet shown in FIG. 1B—the next stage is to assemble the turbine discs (TD) onto the turbine stud (TS) from right to left with a turbine nut (TN) being threaded onto the other end of the turbine stud (TS), as shown in FIG. 1C. The hydraulic tool is applied to stretch the turbine stud (TS) and the nut (TN) is tightened to retain the pre-load or pre-tension when the tool is removed.
It will be appreciated that this is a complicated arrangement which requires careful machining and assembly for adequate operation and a long service life. The material of the stud, the dimensions of the stud, the amount of stretch of the stud, etc. has to be considered to ensure sufficient rotating load at all operating conditions of the gas turbine engine. In particular, the threaded connections and the studs may experience stress.
It has to be noted that with “load” a clamping force in axial direction—or at least with a vector component in axial direction—is meant applied by the stud to the discs.
With “load” also a force is described, due to the pre-load or by rotational movement, which acts upon the stud ends. “Load” is considered to be a vector in opposite direction than the axial component of a vector of the force that acts on the stud ends by the nuts.
“Pre-load” is considered a force that exist in a non-rotational state when all parts are assembled. The pre-load force is present as several rotor parts are connected or secured or clamped together.
It is a goal of an embodiment of the invention to reduce stress and fatigue of the stud and the threads.